[llvm] [CHERI] Add enum values and LL parse/print support for CHERI calling conventions. (PR #156328)

[Owen Anderson via llvm-commits]

resistor wrote:

> These are a CHERIoT thing and should be named as such. I keep meaning to rip them out of CHERI LLVM as they only exist for the original MIPS-based compartmentalisation prototype.

I'll rename and come back to this then.

https://github.com/llvm/llvm-project/pull/156328